In the past, several types of gas turbine engines have been available for powering aircraft. The turbofan and the turboprop are two examples of such engines. The turbofan engine includes a core engine, i.e., a gas generator, for generating combustion gases which are expanded through a power turbine to drive a fan, whereas the turboprop engine includes a gas generator and power turbine which drives a propeller. Conventional turboprop engines differ from turbofan engines in several fundamental respects. For example, turboprop engines typically have a much greater blade diameter than turbofan engines. This allows the blades to move a relatively large mass of air for producing thrust. Furthermore, for a given energy input to the blades, a relatively small velocity increase will be imparted to the air passing therethrough. Small velocity increases translate to high engine propulsive efficiencies. Simply stated, propulsive efficiency is a measure of how much available energy is converted to propulsive force. Large velocity increases to air passing through propulsor blades result in "wasted" kinetic energy and lower propulsive efficiency.
Turbofan engines move a somewhat smaller mass of air than do turboprops for the same energy input and impart a larger velocity component to the air in order to achieve the required thrust. This results in a lower propulsive efficiency. Turbofan engines also include a nacelle radially surrounding the fans. This creates an additional drag on the engine which degrades overall engine efficiency. However, the nacelle defines an inlet which diffuses the airstream entering the fan thereby slowing its speed. In this manner, air enters the fan with relatively low axial velocity which is generally independent of flight speed. Such low axial velocities decrease blade drag losses thereby making higher cruise speeds attainable.
Intermediate-sized transport aircraft, for example, 100 to 180 passenger transports, typically utilize turbofan engines for propulsion. Turbofans provide the relatively high thrust required for powering these aircraft at relatively high altitudes and at cruise speeds of about Mach 0.6 to about Mach 0.8. For aircraft designed for lower cruise speeds, conventional turboprops are typically used inasmuch as they can provide superior performance and efficiency. For example, significant reductions in fuel burn, i.e., the amount of fuel consumed per passenger mile, are possible from the use of the aerodynamically more efficient turboprop over the turbofan.
Frequently, it is desirable to have a gas turbine engine which is capable not only of providing a propulsive thrust but also a vertical lifting thrust or auxiliary mechanical power for operating a generator and other equipment in an aircraft. By "vertical lifting thrust" it is meant that a vertical force is exerted on the aircraft to oppose gravity while a "propulsive force" is understood to mean a force which propels an aircraft in a substantially horizontal direction. In one prior art system, vertical lifting thrust is provided by a gas turbine engine having rotatable fan blades pivotally mounted to an aircraft. The fan blades produce a force parallel to the longitudinal axis of the engine. To produce a lifting force the engine is pivoted with respect to the aircraft such that the longitudinal axis is substantially perpendicular to the ground. As the engine is pivoted parallel to the ground, the propulsive force to the aircraft increases and the lifting force decreases. Aircraft incorporating such systems are frequently called vertical takeoff and landing (VTOL) aircraft.
Alternate means for providing a vertical lifting thrust are known in the art, such as, for example, the provision of propellers or fans which rotate about a vertical axis as are found in helicopters. Shaft turbine engines, which have been used for such vertical lifting, are like turboprop engines. In VTOL aircraft, vertical thrust has been effected in part by lift fans which are driven by exhaust from turbojet propulsion engines which impinges on the fan blading.
In order to provide a vertical lifting thrust in an intermediate-sized transport aircraft a relatively large power output is required. For this purpose it is desirable to have a comparatively more efficient gas turbine engine having significant performance increases over conventional turbofan or turboprop engines. Preferably, such an engine would be directly coupled to propulsive blading as well as vertical lift blading in order to control the balance of propulsive and lifting forces.
In some aircraft applications of gas turbine engines, it is desirable to provide some means of driving special auxiliary equipment directly from the engine, i.e., to provide a high capacity power takeoff from the engine. Such power takeoff may be used to drive generators or alternators to provide electrical power to equipment aboard the aircraft. The ability to provide auxiliary power must be balanced against the need for propulsive thrust from the engine, i.e., the extracted auxiliary power should not detrimentally affect the available thrust from the engine. However, it is also desirable to maximize the available auxiliary power at times when the engine thrust is at minimum or cruise values without significant effect on engine performance. A power takeoff suitable for providing sufficient power could be used to provide a vertical lifting force or could drive a large electric generator to provide a large amount of electric power. It will be recognized that prior art systems of the type described above which use primary thrust mechanisms for generating vertical lifting thrust do not provide for large quantities of auxiliary power. Typically, prior engines extract small amounts of auxiliary power from the gas generator rotor via gears, but this method cannot provide the large amount of power contemplated in this invention due to the overwhelming disturbance to the gas generator operation.
A recent improvement over the engines described above is the unducted fan engine, such as disclosed in U.S. patent application Ser. No. 071,594 - Johnson, filed July 10, 1987. In the unducted fan engine, the power turbine includes counterrotating rotors and turbine blades which drive counterrotating unducted fan blades radially located with respect to the power turbine. In order to achieve optimum performance, each of the unducted fan blades has a variable pitch.
In view of the above-mentioned limitations believed to exist among conventional turboprop and turbofan engines, it is an object of the present invention to provide a single gas turbine engine which more efficiently transfers combustion energy into propulsion as well as vertical lift or auxiliary power than engines known in the prior art. Another object of the present invention is to provide a means for efficiently controlling the distribution of combustion energy into propulsive and auxiliary power. Still another object of the present invention is to provide a means for adjusting the speed of vertical lifting blades in order to impart a relatively large quantity of air and improve the efficiency of lifting. The present invention comprises a relatively simple, reliable and efficient system for providing an aircraft with horizontal propulsive thrust as well as vertical lifting thrust or large amounts of auxiliary power in order to meet aircraft and equipment requirements.
In an illustrative embodiment, the present invention comprises a new and improved gas turbine engine having a gas generator effective for generating combustion gases and means for efficiently transferring the combustion energy into a net engine thrust. The transferring means include a counterrotating power turbine with first and second counterrotating propellers. The power turbine includes a first rotor having a plurality of first turbine blade rows extending radially outwardly therefrom and a second rotor having a plurality of second turbine blade rows extending radially inwardly therefrom. The first and second rotors are arranged so as to define inner and outer flowpath surfaces, respectively, for the combustion gases flowing through the power turbine. The power turbine is effective for receiving the combustion gases and extracting substantially all the output power therefrom for driving the first and second rotors in counterrotating directions. The first and second counterrotating propellers each have a plurality of variable pitch blades attached to first and second rotatable nacelle rings, respectively. The first and second propellers are directly coupled to and driven by the first and second rotors, respectively, and are disposed radially outwardly of the power turbine. Each of the blades has a relatively high hub radius to tip radius ratio and relatively low thickness to chord ratio. The propeller blades are capable of producing a propulsive force in a direction parallel to the longitudinal axis of the engine. A first beveled gear, having an axis of rotation parallel to the engine centerline, is coupled to and driven by the first rotor. A second beveled gear, having an axis of rotation parallel to the engine centerline, is coupled to and driven by the second rotor. A third beveled gear is coupled to and driven by the first and second beveled gears. The third gear is coupled to an auxiliary drive shaft oriented substantially perpendicular to the primary engine axis. The drive shaft may be coupled to mechanically drive an alternator or generator or to drive a plurality of variable pitch lifting fan blades such that rotation of the third gear rotates the lifting fan blades. Rotation of the rotors drives the lifting fan blades as well as the propeller blades. Energy from the rotors may be transferred to the propeller blades and the lifting blades in variable proportions by varying the corresponding pitch of the propeller blades and the lifting blades. Alternately, modulated energy can be transferred to a large generator or fluid pump by adjusting the pitch of the forward propulsive blades. This transferred energy can even exceed the total outpower of the gas generator system by adding to the turbine power the transient power available by setting the propulsive blade pitch to the windmill condition.